SMTP was first implemented when Internet was a network much different than the
one we use today. Relaying back then was not frowned upon, as it was a legitimate
and sometimes necessary way of sending messages to or from hosts that were not
constantly connected. RFC 821 provides even specific commands for relaying,
in the form

MAIL FROM:<>

RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA>

This is the first point we should note -- This syntax should not be allowed
by the wrapper. Current Sendmail versions usually rejects this syntax, but it
should not be overlooked.